Twin-pipe shock absorber

ABSTRACT

A piston in cylinder shock absorber having a hollow piston rod attached to the piston and a central pipe extending within the piston rod and piston receiving limited flow upon stroking of the piston. A first damper valve creates a variable flow resistance from a reservoir when the piston moves outwardly and a second damper valve creates a variable flow resistance of flow through the piston when the piston is displaced inwardly. A regulating mechanism controls each damper valve by control over the limited flow through the central pipe including an electrically operated servo valve varying outflow from the central pipe to the reservoir.

BACKGROUND OF THE INVENTION

The invention relates to a twin-pipe shock absorber, of a known typeincluding an oil reservoir between a working cylinder and an outer pipe,a piston which is displaceable in the working cylinder, and one or morebores provided in the piston which form the connection between thespaces above and below the piston.

A non-return upflow valve is provided which can open these bores on theinward stroke of the piston rod for the displacement of oil from thespace below the piston to the space above the piston. A hollow pistonrod extends towards the top end of the working cylinder. A bottom platecloses the bottom end of the shock absorber, and a disc is fittedbetween the piston and the bottom plate inside the working cylinder,provided with one or more bores. A non-return upflow valve regulates theupgoing flow through the bores in the disc. A central pipe extendsthrough the piston in the hollow piston rod and also through the disc. Aregulated first damper valve is fitted at the bottom end of the workingcylinder, and a regulating mechanism is disposed below the first dampervalve in order to regulate the flow resistance caused by theabove-mentioned first damper valve.

Such a shock absorber is known from international patent application No.PCT/NL 89/00016 in the name of Koni B.V.

The main advantages of this known type of shock absorber are that theshock absorption is essentially independent of the piston speed and thata single regulating mechanism regulates the damping on the ingoing andoutgoing stroke. A disadvantage is, however, that the full stroke volumeof absorber fluid has to be displaced through the central pipe to thedamper valve working in one direction which handles both the damping ofthe outgoing and of the ingoing stroke. At each stroke a large part ofthe cylinder contents flows to the reservoir and back. In particular,flowing back in the correct manner limits the maximum achievable pistonspeed at which the shock absorber still functions in the optimum way,while the dimensions of the piston and the piston rod are also subjectto limitations.

The object of the invention is to avoid these disadvantages and toprovide a shock absorber of the type mentioned above which stillfunctions well at very high piston speeds, and where no limitations needto be imposed as regards the dimensions of the piston and piston rod.

SUMMARY OF THE INVENTION

According to the invention, a twin-pipe shock absorber of the generaltype described has a first damper valve fitted at the bottom end of theworking cylinder which functions only to dampen on the inward stroke ofthe piston rod. A regulated second damper valve is fitted in the pistonfor the purpose of damping the outward stroke of the piston rod, and theregulating pressure for the second damper valve is governed via thecentral pipe of low cross-sectional area by the above-mentionedregulating mechanism.

This design can be made relatively small using miscellaneous standarddimensions and parts.

The regulating mechanism can be in different forms. It is preferably aservomechanism with a relay piston fitted in an exciting chamber whichis provided with a connecting aperture to the oil reservoir, the passageof which aperture is determined by the electro-magnetically regulatedposition of the valve body of a servo valve, while the central pipe isconnected to the exciting chamber at its bottom end by means of a narrowbore in the relay piston of the servomechanism.

As a result of the connection of the bottom end of the central pipe tothe relay piston of the servomechanism, the guidance of the pipe isrelieved of lateral forces, and the relay piston is guided better andprotected against tilting.

The valve body of the servo valve is preferably connected to a coilwhich can be moved up and down and which is situated inside an axialpermanent magnet, with electrical cables and connections connecting thecoil to a control unit which regulates the electric current to the coilin correspondence with a measured control parameter.

In order to be able to excite the coil in a simple manner, the valvebody of the servo valve is urged upwards through leaf springs which alsoform part of the electric connection to the coil.

Different embodiments are possible for the second damper valve fitted inthe piston. In a preferred embodiment, the second damper valve comprisesa slide displaceable in a cylindrical chamber in the piston. Sidechannels in the wall of the cylindrical chamber are adapted to connectthe space above the downward moved slide to the space below the piston.The piston has one or more holes forming a connection between the spaceabove the piston and slide, and the slide has one or more bores forminga connection between the spaces above and below the slide, and the spacebelow the slide is connected to the central pipe by means of one or moreradial bores in the piston rod and the space between the hollow pistonrod and the central pipe.

In a variant, the above-mentioned second damper valve comprises aseparate valve fitted on a slide, which slide is displaceably fitted ina cylindrical chamber in the piston, while one or more side channels arefitted in the wall of the cylindrical chamber, the channels permanentlyconnecting the space below the damper valve to the space below thepiston. The piston has one or more holes forming a connection betweenthe space above the piston and the damper valve, while the space belowthe slide is permanently connected to the central pipe by means of oneor more radial bores in the piston rod via the space between the hollowpiston rod and the central pipe.

The valve body of the first damper valve can be designed in the form ofpre-tensioned spring steel discs whose pre-tensioning is determined bythe relay piston.

The invention will now be explained in greater detail with reference tothe Figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a first embodiment.

FIG. 1A is an enlarged fragmentary view of a section of the piston shownin FIG. 1.

FIG. 1B is an enlarged fragmentary view of a section of the servo valveand associated elements.

FIG. 2 is an enlarged cross-section view of a piston and adjacentcylinder portions of a second embodiment.

DETAILED DESCRIPTION

The embodiment shown in FIG. 1, comprises an inner pipe forming theworking cylinder 1, an outer pipe 2, and oil reservoir 3 situatedbetween the inner and outer pipes, a piston 4 displaceable up and downin the working cylinder, a hollow piston rod 5, a top plate 6A and abottom plate 6B which closes off the ends of the shock absorber, a disc7 fitted at the bottom end of the working cylinder, a central pipe 8projecting though the piston 4 and extending into the hollow piston rod5, and damper valves regulated by a servomechanism which will bedescribed further below.

An outer ring of bores 9 and an inner ring of bores 10 are fitted in thepiston 4. The outer ring of bores 9 forms the connection between thespaces above and below the piston 4. This connection can be opened forupward oil flow (on inward stroke of the piston) by means of anon-return upflow valve 11. Upflow valve 11 covers the bores 9 whenclosed but is open over bores 10 as best seen in FIG. 1A.

A first damper valve in the form of a slide 12 for damping of theoutward stroke is fitted in an annular space 41 in the piston 4. Thespace below the slide 12 is permanently connected to the cavity in thepiston rod 5 by means of radial bores 13 in the piston rod wall. Theinner ring of bores 10 in the piston 4 forms the connection between thespace above the piston 4 and the space above the slide 12. The slide 12is urged upwards by a spring 14 of limited pre-tensioning. A ring ofnarrow bores 15 is formed in the slide 12, aligned with the inner ringof bores 10 in the piston. Formed in the space below the outer ring ofbores 9 are radial apertures 16 in the outer wall of the annular space41 in which the slide 12 moves. Apertures 16 form the connection betweenthe space above the slide 12 and the space below the bores 9 when theslide 12 is moved downwards.

The disc 7 contains a set of bores 17 forming the connection between thespaces above and below the disc 7. The space below the disc 7 isconnected to the oil reservoir 3 by means of channels 18. A non-returnupflow valve 19 is fitted at the top side of the disc 7.

The central pipe 8 projects with clearance through a central hole 20 inthe disc 7 and at its bottom end is connected to a relay piston 21 whichis displaceable in an exciting chamber 22. At the top side, the excitingchamber 22 is connected to the central pipe 8 by means of a narrowchannel 23 in the relay piston 21. The relay piston 21 is urged upwardsby a spring 24 resting on the bottom of the exciting chamber 22.

An opening 25 is provided in the bottom of the exciting chamber 22, thepassage of the opening being determined by the position of a valve body26 of a servo valve. As best seen in FIG. 1B, the valve body 26 ismounted on a disc 26A, which is urged upwards by current-conducting leafsprings 27 which are electrically connected to an electric conductor 28to a connection pin 29, and to a coil 30, which is placed on the disc26A so that it can move up and down inside an axial permanent magnet 31.The connection pin 29 during operation forms the electrical connectionto a control unit 40 which regulates the electric current to the coildepending on external parameters (for example, the frequency of movementof the non-sprung part of a car). Underneath the bottom of the excitingchamber 22 is a space 32 which is connected by means of one or morechannels 33 to the oil reservoir 3.

A second damper valve 34 for the inward stroke is made in the form ofpre-tensioned spring steel discs which can act on a seat on the bottomside of the disc 7.

In the case of an outward damper stroke, in which the piston 4 movesupwards, the shock absorber works as follows:

The shock absorber fluid flows through the bores 10 in the piston 4 tothe top side of the first damper valve in the form of a slide 12. Asmall part of this fluid enters the space below the slide 12 through the15 in the slide 12 and then flows through the radial bores 13 into thespace between the central pipe 8 and the inside wall of the piston rodcavity to the central pipe 8. The low volume flow thus produced can beregulated by the servo valve body 26, the position of which can beadjusted by exciting the coil 30. If the servo valve body 26 is in theclosed position, the pressure in the central pipe and below the slide 12will rise to the outward damping pressure and the slide 12 will moveupwards. The damping is maximum. If the servo valve body 26 is in thefully open position, the low volume flow will be able to move throughthe central pipe 8, the channel 23, the exciting chamber 22, the opening25 in the bottom of the exciting chamber, the space 32 and the channel33 to the oil reservoir 3. Little or no pressure builds up in thecentral pipe 8 and below the slide 12, so that the slide moves downagainst the slack spring 14 and opens the side apertures 16. The oil cannow flow from the space above the piston 4 through the bores 10, thespace above the slide 12 which has moved down, the side apertures 16 andthrough the annular space below the bores 9 and bores 9 to the spacebelow the piston 4. The damping is determined by the pressure drop overthe side apertures 16, and is minimal.

For each position of the servo valve body 26 between the fully openposition and the fully closed position there is a position of the slide12, and thus a pressure drop value across the side apertures 16. Thislast pressure drop value determines the pressure in the annular spaceabove the piston 4, and thus the damping. It will be clear that with anupward moving piston 4, oil is replenished from the reservoir 3 throughthe channels 18, bores 17 and the upflow valve 19 in the space below thepiston.

In the case of an inward damper stroke, oil will be displaced out of thespace below the piston 4 through the bores 9 and the upflow valve 11 tothe space above the piston 4, and a quantity of oil corresponding to thevolume of the piston rod stroke will be displaced through the centralhole 20, the second damper valve 34 and the channels 18 to the oilreservoir 3. Since the pressure above the piston 4 is equal to thepressure below the piston 4, a slight quantity of oil will again flowthrough the inner ring of bores 10, the bore 15 in the slide 12, theradial bores 13, the annular space between the central pipe 8 and theinner surface of the piston rod cavity to the central pipe 8, and fromthe central pipe 8, will reach the exciting chamber 22 through thechannel 23. The pressure below the relay piston 21, and thus thepretensioning of the second damper valve 34, is regulated by regulatingthe pressure in the exciting chamber 22 with the aid of the position ofthe servo valve body 26 to be set by exciting the coil 13. The inwarddamping force is controlled directly in this way.

It will be clear that both the inward and the outward damping isregulated by regulating the position of a single servo valve body 26 byelectrically exciting the coil 30 more or less. During inward andoutward movement of the piston rod 5, oil flows to the exciting chamber22 and the quantity thereof is determined by the damping pressure andthe dimensions of the narrow bore 15 (restriction) in the slide 12.

The coil 30 is suspended by means of the disc 26A and leaf springs 27.The power lines of the permanent axial magnet 31 runs in such a way thatthe coil 30 is moved down or up more or less depending on the excitationcausaused by means of the electrical connections 29, 28, 27. The servovalve body 26 is of such a shape that shutting off the exciting chamber22 to a greater or lesser degree can be regulated accurately.

In FIG. 2 parts with equivalent functions are indicated by the samereference number. The slide 12A is no longer provided with a narrowbore, but the pressure equalization between the cylinder space above thepiston 4A and the space below the slide 12A takes place through a boreor bores 15A in the piston rod 5A, the cavity in the piston rod 5A andthe bores 13 in the piston rod 5A. The side channels 16A have such alarge cross-section that they have no further influence on the damping.Fitted between the top side of the slide 12A and a part of the piston 4Ais a first damper valve 35 comprised of a disc engaged by the top sideof the slide 12A, the damping effect of which on flow is thus determinedby the position of the slide 12A, which in turn is determined by theposition of the servo valve body 26. The channels 16A form a permanentconnection between the space below the damper valve 35 and the spacebelow the outer ring holes 9. The damping of the outward stroke isdetermined by the pressure drop over the first damper valve 35.

The main advantages of the invention are:

The shock absorber works well even at very high piston speeds.

The central pipe 8 has a small cross-section and only small damper oilvolumes (control volume flow) pass through this pipe (for example, 1 ccinstead of 8 cc per 1 cm displacement), and consequently thecross-section dimensions of the piston rod 5 can be reduced to standarddimensions.

The damping characteristics for the inward and outward stroke can becontrolled separately.

An additional advantage of the design shown is that the central pipe 8is coupled to the relay piston 21, so that the guidance of the controlpipe 8 is relieved of lateral forces and the relay piston is guidedbetter, while tilting of the relay piston 21 is prevented.

The essential factor for the inventive idea is that both the dampervalve fitted at the bottom end of the working cylinder for damping theinward stoke of the piston rod and the damper valve fitted in the pistonfor damping the outward stroke of the piston rod can be regulated bymeans of a single regulating mechanism.

The embodiments shown are merely examples. Different variants arepossible. For example, instead of a slide 12 or the disc of the dampervalve 35, other damper valve systems by which the regulating oil flowfrom the space above the piston to the exciting chamber can be regulatedcan be selected. Instead of the damper valve 34 shown, it is alsopossible to use a slide which can shut off, open or partially open oneor more openings in the wall of a guide system. Different variants arealso conceivable for the regulating mechanism.

I claim:
 1. A twin-pipe shock absorber, comprising:an outer pipe;including a working cylinder disposed within said outer pipe, saidworking cylinder closed off at either end by respective top and bottomplates; a reservoir space between said working cylinder and said outerpipe; a piston displaceable within said working cylinder; one or morebores extending through said piston defining a flow path between a spaceabove and a space below said piston in said working cylinder; anon-return upflow valve opening said one or more bores on the inwardstroke of said piston receiving a displacement of oil from said spacebelow said piston to said space above said piston; a hollow piston rodextending from the top side of said piston through said top plate ofsaid working cylinder; a disc fitted between said piston and said bottomplate inside said working cylinder, having one or more boresestablishing communication between said reservoir space and said spacebelow said piston; a non-return upflow valve for regulation of theupgoing flow through said bores in said disc; a central pipe extendingthrough said piston and spaced within said hollow piston rod and throughsaid disc; limited flow passage means in said piston causing limitedflow into said central pipe upon outward displacement of said piston insaid working cylinder; a damping flow passage means in said pistonallowing flow from said space above said piston to said space below saidpiston upon upward movement of said piston; a variable flow resistancefirst damper valve means interposed in a flow path fitted at the bottomend of said working cylinder between said space below said piston andsaid reservoir space; and, a variable flow resistance second dampervalve means in said piston controlling the resistance to flow throughsaid damping flow passage means in correspondence to said limited flowin said central pipe; a regulating mechanism below said first dampervalve means regulating said resistance to flow of said first dampervalve means and also controlling said limited flow in said control pipeto control said second damper valve means.
 2. The twin-pipe shockabsorber according to claim 1, wherein said regulating mechanismcomprises a servomechanism having a relay piston, an exciting chamberreceiving said relay piston, a connecting aperture adapted to bevariably opened communicating with said reservoir, the extent of openingof said aperture determined by a servo valve including anelectro-magnetically positioned valve body, said central pipe connectedto said exciting chamber by a narrow bore in said relay piston of saidservomechanism.
 3. A twin-pipe shock absorber according to claim 2,wherein said central pipe is fixed at its bottom end to said relaypiston of said servomechanism.
 4. A twin-pipe shock absorber accordingto claim 2, wherein said valve body of said servo valve is connected toan electromagnetic coil movable up and down, and further including anaxial permanent magnet receiving said movable coil, and a control unitmeans regulating electrical current to said coil.
 5. A twin-pipe shockabsorber according to claim 4, wherein said valve body of said servovalve is urged upwards by one or more leaf springs, said leaf springscomprising an electrical connection to said coil.
 6. A twin-pipe shockabsorber according to claim 2, wherein said second damper valve meanscomprises a slide, a annular space in said piston displaceably receivingsaid slide, said annular space defined by a wall having one or more sideaperatures adapted to connect a space above said slide to said spacebelow said piston, said piston having one or more holes connecting saidspace above said piston and said space above said slide, said slidehaving one or more bores forming a connection between said spaces aboveand below said slide, and that said space below said slide ispermanently connected to said central pipe by means of one or moreradial bores in said piston rod and a space between said hollow pistonrod and said central pipe.
 7. A twin-pipe shock absorber according toclaim 2, wherein said second damper valve means comprises an annularspace in said piston, said slide displaceable in said annular space, oneor more side apertures in a wall of said annular space, said aperturespermanently connecting a space below said second damper valve means tosaid space below said piston, said piston having one or more holesforming connections between said space above said piston and above saidsecond damper valve means, and in that a space below said slide isconnected to said central pipe by means of one or more radial bores insaid piston rod and said central pipe.
 8. A twin-pipe shock absorberaccording to claim 2, wherein said first damper valve means is comprisedof pretensioned spring steel discs, the pretensioning of said springsteel discs controlled by said relay piston.
 9. A shock absorbercomprising:an outer cylinder; an inner working cylinder disposed withinsaid outer cylinder with a space therebetween defining a reservoir for adamping fluid; first end plate means closing off one end of said outercylinder and said working cylinder; second end plate means closing offthe other end of said outer cylinder; a disc fit into the other end ofsaid working cylinder; a piston slidably displaceable in said workingcylinder defining a first space in said working cylinder between saidpiston and said first end plate means and a second space in said workingcylinder between said piston and said disc; a hollow piston rod fixed tosaid piston and extending from one side thereof through said first endplate means; a first damper valve means in said disc allowing a variableresistance flow from said second space into said reservoir upondisplacement of said piston towards said disc; a central pipe extendingwithin said hollow piston rod and said piston through said disc; controlvalve means controllably communicating the interior of said central pipewith said reservoir; a limited flow path receiving limited fluid flowfrom said first space and into said central pipe with said pistondisplaced towards said first end plate means; a damping flow path insaid piston allowing fluid flow from said first space to said secondspace when said piston is displaced towards said first end plate means;a second damper valve means in said piston allowing a variableresistance to flow through said damping flow path upon displacement ofsaid piston towards said one end of said working cylinder; and,regulating means controlling said second damper valve means by controlover said limited flow.
 10. The shock absorber according to claim 9wherein said regulating means further includes means also controllingsaid first damper valve means by said limited flow, whereby said firstand second damper valve means are both controlled by control over saidlimited fluid flow.
 11. The shock absorber according to claim 10 whereinsaid regulating means further comprises a servomechanism which includesservo valve means for controlling said limited fluid flow.
 12. The shockabsorber according to claim 11, wherein said regulating means includesan exciting chamber defined in said second end plate means, connectingsaid central pipe with said exciting chamber to receive said limitedflow; said servo valve means comprising means controlling outflow fromsaid exciting chamber to said reservoir.
 13. The shock absorberaccording to claim 12 wherein said servo valve means includes an openinginto said exciting chamber and a servo valve body variably positionedrelative to said opening to control flow through said opening as afunction of said relative position thereof; and electrical operatingmeans variably positioning said servo valve body relative to saidopening.
 14. The shock absorber according claim 13 further including arelay piston slidable into said exciting chamber, said central pipemounted in said relay piston and an internal passage in said relaypiston connecting said central pipe with said exciting chamber, aclearance in said disc exerting fluid pressure in said second space onsaid first damper valve means, said first damper valve means operated bymovement of said relay piston under the influence of said fluid pressurethereon, whereby outflow from said exciting chamber controls said firstdamper valve means.
 15. The shock absorber according to claim 13 whereinsaid servo valve means includes leaf springs drivingly connected to saidservo valve body and an electro-magnetic coil mounted to be movable withsaid valve body and leaf springs.
 16. The shock absorber according toclaim 11 wherein said second damper valve means comprises a slidemovably mounted in said piston, and means varying said fluid flowresistance through said piston in correspondence to the position of saidslide and means varying said slide position in accordance with saidlimited fluid flow.
 17. The shock absorber according to claim 16 whereinsaid second damper means comprises a metering opening in said pistonvariably covered by said slide, a restricted opening through said slidecommunicating with said central pipe, said slide variably positioned bychanges in pressure in said central pipe by variations in said limitedflow.
 18. The shock absorber according to claim 16 wherein said seconddamper means comprises a disc engaged by one side of said slide to exerta force thereon, the other side of said slide having the pressure insaid central pipe exerted thereon to generate a variable forcecorresponding to said limited fluid flow.